In general, pyrolysis is a thermal degradation process in which large molecules are broken or cracked into smaller molecules in the presence of little if any oxygen. A wide variety of hydrocarbon materials can be pyrolyzed to produce vapor, liquid and solid materials into more readily useable forms. Pyrolysis should not be confused with other processes that use both heat and additional chemical reactivity to alter molecular structure, which processes usually take place in reactive (non-inert) atmospheres, e.g., hydroprocessing/hydrotreatment in the presence of hydrogen-containing gas, sulfiding in the presence of a sulfur-containing gas, and the like.
Pyrolysis has been used to produce high viscosity tars from biomass materials for centuries. More recently, pyrolysis technology has been developed to increase yield and quality of vapor and liquid products.
U.S. Patent Application Publication No. 2009/0151253 discloses methods and systems to convert carbonaceous materials (such as biomass) into synthesis gas and other downstream products (such as alcohols). In certain embodiments, pyrolysis is performed in the presence of a catalyst such as heterogeneous catalysts (such as SiO2—Al2O3, Pt/SiO2—Al2O3, WOx/ZrO2, SOX/ZrO2), zeolites (such as HY-zeolite, alpha-zeolite, HZSM-5, ZSM-5, or klinoptilolite), acid catalysts, clay catalysts (e.g., acidified or activated clay catalysts), Al-MCM-41 type mesoporous catalysts, activated alumina, CoMo catalysts (such as Criterion-534), and Ni/Al co-precipitated catalysts. In some embodiments, a cation such as K+, Li+, or Ca2+ can be used to increase the selectivity and yield of char and/or to lower the selectivity and yield of tar during pyrolysis.
U.S. Patent Application Publication No. 2009/0165378 discloses processes for fractional catalytic pyrolysis of biomass materials. The processes involve the use of a suitable catalyst in a fluidized bed pyrolysis system. The catalyst is H-ZSM-5, an aluminosilicate zeolite catalyst. Super acid catalysts, such as sulfated zirconia super acid catalysts, can also be used.
In spite of the advances in pyrolysis production, problems still exist with product quality, such as stability of the products produced, particularly in the case of pyrolysis liquid products produced from significant quantities of biomass materials. Over time, the viscosity of pyrolysis liquid products tends to increase as a result of polymerization reactions taking place. Heating the liquids (i.e., the condensed pyrolysis liquids) accelerates the polymerization reactions, in particular the reaction between formaldehyde and phenol, which are typically present in appreciable quantities. This instability makes transportation and long term storage of pyrolysis liquids a challenge.